J. Hellmig

Exclusion limits on the WIMP-nucleon cross section from the cryogenic dark matter search.

The Cryogenic Dark Matter Search (CDMS) employs Ge and Si detectors to search for WIMPs via their elastic-scattering interactions with nuclei while discriminating against interactions of background particles. CDMS data give limits on the spin-independent WIMP-nucleon elastic-scattering cross-section that exclude unexplored parameter space above 10 GeV c^{-2} WIMP mass and, at>84% CL, the entire 3

Exclusion limits on the WIMP nucleon cross-section from the cryogenic dark matter search

sigma

A solution to the dead-layer problem in ionization and phonon-based dark matter detectors

allowed region for the WIMP signal reported by the DAMA experiment.

Identification of single-site events in germanium detectors by digital pulse shape analysis

The Cryogenic Dark Matter Search (CDMS) employs low-temperature Ge and Si detectors to search for Weakly Interacting Massive Particles (WIMPs) via their elastic-scattering interactions with nuclei while discriminating against interactions of background particles. For recoil energies above 10 keV, events due to background photons are rejected with>99.9% efficiency, and surface events are rejected with>95% efficiency. The estimate of the background due to neutrons is based primarily on the observation of multiple-scatter events that should all be neutrons. Data selection is determined primarily by examining calibration data and vetoed events. Resulting efficiencies should be accurate to about 10%. Results of CDMS data from 1998 and 1999 with a relaxed fiducial-volume cut (resulting in 15.8 kg-days exposure on Ge) are consistent with an earlier analysis with a more restrictive fiducial-volume cut. Twenty-three WIMP candidate events are observed, but these events are consistent with a background from neutrons in all ways tested. Resulting limits on the spin-independent WIMP-nucleon elastic-scattering cross-section exclude unexplored parameter space for WIMPs with masses between 10-70 GeV c^{-2}. These limits border, but do not exclude, parameter space allowed by supersymmetry models and accelerator constraints. Results are compatible with some regions reported as allowed at 3-sigma by the annual-modulation measurement of the DAMA collaboration. However, under the assumptions of standard WIMP interactions and a standard halo, the results are incompatible with the DAMA most likely value at>99.9% CL, and are incompatible with the model-independent annual-modulation signal of DAMA at 99.99% CL in the asymptotic limit.

Exclusion limits on the WIMP-nucleon scattering cross-section from the Cryogenic Dark Matter Search

Abstract We report on a study of several different electrode technologies to avoid the “dead-layer” problem in ionization and phonon-based dark matter detectors. We have found the most success with an amorphous blocking-layer electrode structure, and have demonstrated background electron rejection of ≈95% above 20xa0keV.

The CDMS II Z-sensitive ionization and phonon germanium detector

Abstract A method that discriminates between single- and multiple-site interactions in germanium detectors was developed. For this purpose the output of customary charge-sensitive preamplifiers was differentiated with 10–20xa0ns sampled with 250xa0MHz and analyzed off-line. Multiple-site interactions were measured in the 1621xa0keV photo peak and single-site events in the 1592xa0keV double escape line of 208 Tl. One single parameter describing the broadness of the charge pulse maximum is sufficient to achieve 80% detection efficiencies for both the interaction types, over a wide energy range. As examples, employment of the method in the Heidelberg–Moscow experiment resulted in the consistent half-life of two neutrino double beta decay and the reduction of the background by a factor of three in searches for the neutrinoless double beta decay. Identification of single Compton scattered events improved the minimal detectable activity of a 25% efficiency detector by a factor of two.

Deployment of the first CDMS II ZIP Detectors at the Stanford Underground Facility

Abstract The Cryogenic Dark Matter Search (CDMS) employs massive ionization- and phonon-mediated detectors to search for WIMPs via their elastic scattering interactions with nuclei while discriminating against interactions by other background particles. Limits on the WIMP-nucleon scattering cross-section, based on 3.1xa0kgxa0d of exposure, exclude new parameter space in the 10–30xa0GeV WIMP mass region and also a portion of the region allowed by the DAMA annual modulation search (Bernabei, Phys. Lett. 450 (1999) 448).

Position dependence in the CDMS II ZIP detectors

Abstract The last year saw major progress in the development of the nuclear-recoil dark matter detector for the Cryogenic Dark Matter Search (CDMS II) experiment. The basic γ-ray background discrimination is based on ionization yield. However, as shown in separate experiments, surface events and especially electrons also results in low ionization yield due to incomplete charge collection. The two-fold strategy to reduce these unwanted backgrounds was to improve charge collection at the detector surface and to employ fast phonon sensors on Ge. A 250xa0g Ge detector with an Al Schottky contact on an amorphous Si blocking layer and a Quasiparticle-trap-assisted Electrothermal-feedback Transition-edge (QET) phonon sensor has been characterized. The phonon collection efficiency of the new detector is similar to that of the latest 100xa0g Si Fast Large Ionization and Phonon (FLIP) detector, which has the same phonon sensor design. The fall times of the phonon pulses are longer and consistent with simulations. The charge collection at the surface is high and very similar to that of smaller test devices and the last revision of Berkeley Large Ionization and Phonon (BLIP) detectors. The four fast non-thermal phonon sensors yield x, y position resolution similar to the Si FLIP detector. A rise time surface effect was demonstrated on both sides of the detector. The effect results in a much more effective rejection of surface events than the rejection based on charge yield alone and therefore improves the sensitivity for a dark matter search.

Charge collection and electrode structures in ionization and phonon based dark matter detectors

Abstract The CDMS II experiment deployed the first set of ZIP (Z-dependent Ionization and Phonon) detectors at the Stanford Underground Facility (SUF) shallow depth site in the spring of 2000. With a payload consisting of 3 Ge (250g ea.) and 3 Si (100g ea.) ZIPs, the run was the first demostration of multiple ZIPs operating simltaneously. Good discrimination between electron and nuclear recoil events of 99.8% was established, down to recoil energies of 10 keV. A measurement of the γ, β, and neutron backgrounds was made.

Demonstration of the CDMS II ZIP technology at a shallow underground site

The Ge and Si detectors developed by the Cryogenic Dark Matter Search (CDMS) II experiment rely on the simultaneous detection of athermal phonons and ionization produced by interactions in the crystal. The athermal phonons provide both the total energy deposited in an interaction and the information about the position of the interaction. We describe extracting this information from the pulse shapes in the four phonon sensors. We present the result of measurements made on a Si detector from the first CDMS II production batch. We also investigate ways of using the event position information to extract further information about the phonon signal.